Suspensions for motor vehicles are devices used to connect a vehicle body and wheels to each other, and are adapted to absorb shock and vibration applied from the road during movement, thereby improving ride comfort and stability of motor vehicles. Such suspensions are basically classified into integral type suspensions wherein left and right wheels are connected by an axle, and independent type suspensions wherein left and right wheels operate individually.
The suspensions comprise springs for absorbing shock from the road, shock absorbers adapted to restrict free vibration of the springs, arms or links for controlling the operation of wheels, and the like.
There is another type of suspension—torsion beam type suspension—configured in a fashion such that a pair of left and right trailing arms are coupled to each other by using a single member called a torsion beam or cross beam. Such configuration of the torsion beam type suspension is characterized in that the length of its link is longer, and the number of rubber bushes functioning as oscillating shafts is smaller, compared with strut type or double wishbone type suspensions. The torsion beam type suspension has several advantages in that it shows low friction hysteresis in relation to a suspension stroke, induces a high level of smooth ride comfort, and achieves a relatively high driving stability relative to its low production cost and mass, in spite of the fact that its design performance range is not high due to the simplification of constitutive components thereof. Thus, the torsion beam type suspension has been used in small and sub-medium sized motor vehicles during the last several decades.
Referring to FIG. 1, which illustrates a conventional torsion beam type suspension, a pair of left and right trailing arms 2 and 2′ are connected by a torsion beam 4, the front ends of the trailing arms 2 and 2′ serving to pivotally support a vehicle body (not shown) by joints 6 and 6′ having rubber bushes, respectively, and the rear ends of the trailing arms 2 and 2′ are coupled with wheels 8 and 8′. Suspension springs 10 and 10′ are respectively installed between the trailing arms 2 and 2′ and the vehicle body, and shock absorbers 12 and 12′ are respectively connected to the rear ends of the trailing arms 2 and 2′. The conventional torsion beam type suspension constructed as stated above has features that enable the wheels 8 and 8′ to be deformed due to the torsional deformation property of the torsion beam 4, and results in a toe-in state in case of bumps by virtue of such torsional deformation, position of the trailing arms, and a specific bushing property. The toe-in state means a state wherein, when looking down at the wheels, the distance between the front portion of the wheels aligned on the same axle is shorter than the distance between the rear portion of the wheels.
In the torsion beam type suspension as stated above, the trailing arms and torsion beam are welded together, thereby defining a so-called torsion beam axle. When a pair of left and right wheels move in opposite directions, the torsion beam axle is partially or wholly twisted. Such torsional twisting considerably affects the suspension, and is an important factor for controlling the performance of a motor vehicle. In this regard, the torsion beam should have a high torsional twisting rigidity against the rolling of a motor vehicle and a high bending rigidity against a lateral force inputted through tires when the motor vehicle makes a turn.
The conventional torsion beam, however, is manufactured by forming a thick iron plate having a thickness of about 4 mm to 5 mm to have a cross-sectional shape of a letter U by making use of a press, as shown in FIG. 1. Since the conventional torsion beam has insufficient twisting rigidity, bending rigidity and durability thereof, a separate torsion bar for satisfying a required twisting rigidity or bending rigidity, and a reinforcement for satisfying a required durability should be welded to the torsion beam.
The conventional torsion beam configured as stated above inevitably complicates the overall assembly process due to an increase in the number of constitutive components, and consequently increases the overall weight of a product.